a) Field of the Invention
This invention relates to a wet multidisk friction clutch, and especially to the construction of a driving friction disk formed of a disk-shaped metal core and friction linings bonded on opposite sides of, said disk-shaped metal core and also to a process for the fabrication of said disk-shaped metal core.
b) Description of the Related Art
The basic construction of a wet multidisk friction clutch is shown in FIG. 5. FIG. 6 is a cross-sectional view corresponding to a view taken in the direction of arrows VI--VI of FIG. 5 and shows the construction of a conventional clutch drum. FIG. 7A and FIG. 7B illustrate the construction and arrangement of conventional friction disks. In FIG. 5, there are shown a clutch drum 1, driven friction disks 2, a clutch hub 3, driving friction disks 4, a piston 6, an engagement member 9a, and a cylindrical shaft 14.
FIG. 7A illustrates the construction and arrangement of the driven friction disks 2 and driving friction disks 4, whereas FIG. 7B shows the friction plates 2,4 in a stacked and pressed position. Each driven friction disk 2 is formed of a metal disk with spline tabs 21 arranged on an outer periphery thereof.
Each driving friction disk 4 is formed of a disk-shaped metal core 4b having spline tabs 41 arranged on an inner periphery thereof and wet friction linings 4a bonded on opposite sides of the disk-shaped metal core 4b.
FIG. 6 shows, as mentioned above, the conventional construction of the clutch drum 1, taken in the direction of arrows VI--VI of FIG. 5. The clutch drum 1 is constructed of an outer drum 8 and an inner drum 9. Designated at numeral 10 is a spline groove, with which the corresponding spline tab 21 of the driven friction disk 2 is maintained in engagement. Symbol 8a (see FIG. 5) indicates an oil supply hole.
The inner drum 9 is united by spot weld or the like to the outer drum 9 on a side of the piston 6.
Operation of the friction clutch will hereinafter be described with reference to FIG. 5. Alternately mounted on the cylindrical shaft 14, which is fixed on an unillustrated transmission case, are the driven friction disks 2 supported on the clutch drum 1 and the driving friction disks 4 supported on the clutch hub 3. These friction disks 2,4 are located between the clutch drum 1 supported for rotation and the clutch hub 3 fixed on another clutch drum (not shown).
When the friction disks 2,4 are pressed by the piston 6 against the engagement member 9a, power is transmitted from the clutch hub 3 to the clutch drum 1.
In a friction clutch, it is important to reduce drag torque and also to improve heat-resistant durability.
A groove pattern according to a conventional example, which reduces drag torque, is shown in FIG. 8A. Incidentally, FIG. 8A illustrates the driving friction disk 4 while FIG. 8B depicts the driven friction disk 2. In the driving friction disk 4, the friction linings 4a are bonded on the opposite sides of the disk-shaped metal core 4a (the spline tabs 41 and their adjacent portions are also observed). Oil grooves 4c are formed in a surface of each friction lining 4a, thereby reducing the shear resistance of a lubricating oil and also decreasing the drag torque.
Such a conventional wet multidisk friction clutch is accompanied by the problem that due to formation of a lubricating oil film, friction characteristics and idling-time drag torque vary (especially in a low temperature range in which the lubricating oil has high viscosity). To cope with this problem, it may be contemplated to form oil grooves in a surface of each friction lining as described above. An unduly high surface area percentage of oil grooves however leads to a problem in durability. Further, wearing of the friction lining results in a reduction in the cross-sectional area of each groove decreases, leading to another problem that the effect of the groove is reduced.
When exposed to heat of at least a certain quantity, more energy is absorbed per unit area than the cooling effect of such grooves. Thermal deterioration of the friction linings hence proceeds so that the friction linings may be caused to burn eventually. Accordingly, the reduction in drag torque and the heat-resistant service life of friction linings have the relationship that they conflict with each other with a certain inflexion point.
Further, the state of lubrication under continuous sliding, that is, the durability of the friction linings is significantly affected by the total cross-sectional area (volume) of oil grooves. To improve the heat-resistant service life, it is therefore important to supply the lubricating oil as much as possible. It is however impossible to achieve any sufficient groove depth (cross-sectional groove area) when the grooves are formed by molding or the like. Therefore, the grooves are formed to the metal core by cutting or the like. This however results in substantial cutting of fibers and the like which are supposed to maintain the strength of the friction linings. Accordingly, the durability of the friction linings is deteriorated. Moreover, a cutter may reach even the metal core due to a variation in machining so that the strength of the metal core may be reduced.